This invention deals with a method of electrically isolating portions of a large surface area photovoltaic device for a plurality of purposes such as the production of smaller surface area semiconductor devices. Generally, the invention contemplates the use of electrically conductive grid patterns applied on the transparent conductive layer to divide the semiconductor body of the photovoltaic device into a plurality of isolated portions. Each individual isolated portion is tested for electrical output and those isolated portions providing unsatisfactory electrical output are electrically isolated from the isolated portions providing satisfactory electrical output. Thus, areas of a photovoltaic device not measuring up to preselected standards of electrical output are identified and isolated so as not to interfere with the operation of the remaining portions of the device. The isolation method may also be used to cut small surface area semiconductor devices from larger surface area semiconductor devices, and to improve the electrical output of unsatisfactory isolated portions of the semiconductor body.
Patterns are commonly etched onto the surface of photovoltaic cells and semiconductors through the use of photoresist films. Typically, a photoresist solution is applied to the surface of the semiconductor body and the solvent is removed, thus leaving a thin film as a residue. A grid or circuit pattern of some sort, masking part of the film, is placed over the film, and those portions of the film not covered by the pattern are exposed to ultraviolet electromagnetic radiation or to a beam of electrons of appropriate energy. During development of the film, employing conventional procedures, either the exposed or the unexposed portions of the film are removed, and the pattern is etched through the transparent conductive oxide layer of the semiconductor. The remainder of the photoresist is removed and the grid pattern is applied onto the surface of the isolated portions of the transparent layer. In the course of the processing, drying and curing steps are conventionally performed in air, pursuant to a selected time-temperature regimen.
Recently, considerable effort has been expended to develop processes for depositing amorphous semiconductor alloy layers which may be of relatively large surface area and which may be readily doped to form p-type and n-type materials. These amorphous semiconductors are used for p-n junction operationally equivalent to those produced by their crystalline counterparts. Amorphous silicon or germanium (Group IV) films were found to have microvoids and dangling bonds and other defects which produce a high density of localized states in the energy gap thereof. The presence of a high density of localized states in the energy gap of amorphous silicon semiconductor films results in a low degree of photoconductivity and short carrier lifetime, making such films unsuitable for photoresponsive applications. Additionally, such films cannot be successfully doped or otherwise modified to shift the Fermi level close to the conduction or valence bands, making them unsuitable for p-n junctions for solar cell applications.
Amorphous silicon alloys have now been prepared with significantly reduced concentrations of localized states in the energy gaps thereof and of high electronic quality. However, some defects still exist in the semiconductor films which lowers the efficiency of the photoresponsive device. Similarly, crystalline semiconductor materials suffer from defects in the crystalline lattice. Certain areas of the lattice may have a high density of localized states which would decrease the efficiency of any photoresponsive device, particularly solar cells.
Defects in portions of a semiconductor device may also cause electrical shorting, thereby rendering at least portions of the semiconductor body electrically inoperative. Depending upon the location of the defective portion of the semiconductor body and the severity of the short, the electrical output of the entire semiconductor body may be significantly decreased. It is therefor advantageous to identify those defective portions of the semiconductor body so that those electrically defective portions can be insulated or isolated from the electrically operative portions thereof. This is particularly valuable for large area amorphous semiconductor bodies where there is a long carrier path and the probability of defective portions increases. When only electrically operative portions of the semiconductor device are electrically connected, the total electrical output of the semiconductor device is maximized and overall efficiency increases.
The many objects and advantages of the present invention will become clear from the drawings, the detailed description of the invention and the claims which follow.